1. Field of the Invention
The present invention relates to a thermal transfer printer of the type in which a desired ribbon cassette is selected from a plurality of ribbon cassettes respectively held by a plurality of cassette holding portions of a cassette holder and in which the selected ribbon cassette is transferred to a cassette platform to perform desired printing.
2. Description of the Related Art
In a common thermal transfer printer, paper and an ink ribbon are arranged before a platen, and, while moving a thermal head mounted on a carriage in a printing column direction in a condition in which the thermal head is held in press contact with the platen through the intermediation of the ink ribbon and the paper, a plurality of heat generating elements are selectively caused to generate heat in accordance with printing signals, whereby the ink of the ink ribbon is melted and transferred to the paper to thereby effect printing.
Due to its high printing quality, low noise, low cost, ease of maintenance, etc., such a thermal printer is widely used as an output apparatus for a computer, a word processor, or the like.
To make such a thermal transfer printer more convenient to use and achieve a reduction in running cost, there is known, as has been proposed in U.S. Pat. No. 5,538,351, a thermal transfer printer in which there is provided a cassette holder holding a plurality of ribbon cassettes containing inks of different colors in cassette holding portions. The cassette holder is arranged opposite to the carriage, and a ribbon cassette having an ink of a desired color is selected from the ribbon cassettes held by the cassette holding portions of the cassette holder. The selected ribbon cassette is transferred from the cassette holding portion to a cassette platform arranged on the carriage to thereby make it possible to perform recording by the thermal head.
FIG. 19 schematically shows the construction of the essential part of a cassette holding portion of the cassette holder and the cassette platform of such a conventional thermal transfer printer. In FIG. 19, a ribbon cassette 11 is held by the cassette holding portion 20, and no ribbon cassette 11 is attached to the cassette platform 22.
As shown in the drawing, the cassette holding portion 20 is equipped with a pair of arms 20a and 20b for holding the ribbon cassette 11. Around the base portions of these arms 20a and 20b, torsion coil springs (not shown) are wound so that the ribbon cassette 11, held by the cassette holding portion 20, may be reliably held due to the resilient force of the springs, whereby the ribbon cassette 11 is prevented from being detached from the cassette holding portion 20.
On the side opposite to the cassette holding portion 20 of the cassette holder, there is arranged a carriage 21, on which there is mounted a cassette platform 22 as shown in FIG. 19 so as to be capable of being brought into contact with and moved away from the cassette holding portion 20. A pair of arms 22a and 22b are arranged on either end of the cassette platform 22. Torsion coil springs (not shown) are wound around the base portions of these arms 22a and 22b so that the ribbon cassette 11 placed on the cassette platform 22 may be constantly held due to the resilient force of these springs, whereby the ribbon cassette 11 is prevented from being detached from the cassette platform 22.
The distance between the forward ends of the arms 22a and 22b when the ribbon cassette 11 is not placed on the cassette platform 22 is smaller than the distance between the forward ends of the arms 20a and 20b of the cassette holding portion 20 when the ribbon cassette 11 is being held by the cassette holding portion 20. Further, the distance between the forward ends of the arms 20a and 20b when the ribbon cassette 11 is not being held by the cassette holding portion 20 is smaller than the distance between the forward ends of the arms 22a and 22b of the cassette platform 22 when the ribbon cassette 11 is placed on the cassette platform 22 (See FIG. 19).
Next, the operation of transferring the ribbon cassette 11, held by the cassette holding portion 20 of the thermal transfer printer constructed as described above, to the cassette platform 22, will be described.
First, as shown in FIG. 19, the cassette platform 22 is moved to a position opposite to the cassette holding portion 20 holding the ribbon cassette 11. In this condition, the cassette platform 22 is moved by a driving means (not shown) so as to approach the cassette holding portion 20. As described above, the distance between the forward ends of the arms 22a and 22b of the cassette platform 22 is smaller than the distance between the forward ends of the arms 20a and 20b of the cassette holding portion 20 when the ribbon cassette 11 being is held by the cassette holding portion 20. Thus, the forward ends of the arms 22a and 22b get inside the arms 20a and 20b of the cassette holding portion 20, and, while moving the arms 20a and 20b apart from each other against the resilient force of the torsion coil springs, enter the cassette holder 20. When the cassette platform 22 further moves toward the cassette holding portion 20, the ribbon cassette 11, which has been held by the arms 20a and 20b of the cassette holding portion 20, is elastically held between the arms 22a and 22b of the cassette platform 22 (See FIG. 20). In this way, the ribbon cassette 11 is transferred from the cassette holding portion 20 to the cassette platform 22.
Next, the cassette platform 22, which has thus received the ribbon cassette 11, is brought back to the carriage 21, whereby the desired ribbon cassette 11 is mounted on the carriage 21, thereby making it possible to perform printing by using this ribbon cassette 11 (See FIG. 21).
When the printing by using the desired ribbon cassette 11 has been completed, the ribbon cassette 11 is transferred from the cassette platform 22 to the cassette holder 20. This transfer operation will be described below.
First, as shown in FIG. 21, the cassette platform 22 mounted on the carriage 21 is moved to a position opposite to the cassette holding portion 20 of the cassette holder holding no ribbon cassette.
Then, the cassette platform 22 is moved toward the cassette holding portion 20 by a driving mechanism (not shown). Then, the forward ends of the arms 22a and 22b of the cassette platform 22 are positioned outside the forward ends of the arms 20a and 20b of the cassette holding portion 20 (See FIG. 22).
In this condition, the cassette platform 22 is moved toward the cassette holding portion 20. Then, as shown in FIG. 23, the arms 20a and 20b of the cassette holding portion 20 move while outwardly moving the arms 22a and 22b of the cassette platform 22 apart from each other, and the ribbon cassette 11 is transferred from the cassette platform 22 to the cassette holding portion 20.
Next, the cassette platform 21 with no ribbon cassette 11 mounted thereon is brought back to the carriage 21 (FIG. 24).
When the ribbon cassette 11 has been transferred and the cassette platform 22 with no ribbon cassette 11 mounted thereon has been returned to the carriage 21, the ribbon cassette 11 for next printing is selected, and the above operations of FIGS. 19 through 21 are conducted again, whereby a ribbon cassette 11 accommodating an ink ribbon of another color is transferred from the cassette holding portion 20 to the cassette platform 22.
In such a conventional thermal transfer printer, to reduce the load of the take-up bobbin when taking up the ink ribbon during printing, the frictional resistance between the reels cassette case, i.e., between the supply reel, around which the ink ribbon is wound, and the take-up reel, and the ribbon cassette case supporting these reels, is made as small as possible, whereby the torque of a motor or the like constituting the driving source for rotating the take-up bobbin is reduced to thereby achieve a reduction in power consumption.
However, in this conventional thermal transfer printer, the arms 20a and 20b of the cassette holding portion 20 and the arms 22a and 22b of the cassette platform 22 are formed of materials different from those of the cassette holding portion 20 and the cassette platform 22, respectively. Further, since torsion coil springs (not shown) for elastically holding the ribbon cassette 11 are wound around the arms 20a, 20b, 22a and 22b, the number of parts is rather large, resulting in the cost of the thermal transfer printer being rather high. Further, the large number of parts leads to a poor assembly efficiency.
Further, when the ribbon cassette 11 is attached to or detached from the cassette holding portion 20 or the cassette platform 22 for transfer, any positional deviation of the forward ends of the arms 20a, 20b, 22a and 22b may make it impossible for the ribbon cassette 11 to be transferred.
For example, there is no problem when, as shown in FIG. 19, the positioning of the cassette platform 22 and the cassette holding portion 20 is effected accurately. However, when the right-hand arm 22a of the the cassette platform 22 is deviated to the outside of the right-hand arm 20a of the cassette holding portion 20, or when the left-hand arm 22b of the the cassette platform 22 is deviated to the outside of the left-hand arm 20b of the cassette holding portion 20, it is impossible for the cassette platform 22 to correctly transfer the ribbon cassette 11, so that it is necessary to provide a positioning sensor (not shown) to effect accurate positioning, resulting in a higher cost.
Further, when transferring the ribbon cassette 11 from the cassette holding portion 20 to the cassette platform 22 or from the cassette platform 22 to the cassette holding portion 20, the cassette holder 20 vibrates, so that the ribbon cassette 11 held by the cassette holding portion vibrates, with the result that the above-mentioned reels make some idle running, thereby causing slackness in the ink ribbon wound around the reels. When this ribbon cassette 11, in which the ink ribbon have become slack, is transferred from the cassette holding portion 20 to the cassette platform 22, the slack ink ribbon may get caught by the platen, the upper portion of the thermal head, etc., so that, when the thermal head is lowered when starting printing, the ink ribbon will get wrinkled, thereby making it impossible for printing to be effected in an appropriate manner.